Computer systems execute instructions of various codes. This code involves the transfer of code and data within and between processors and memory devices. Often, the code can be compromised, suffering the introduction of one or more errors. A soft error is a signal or data which is wrong, but does not necessarily imply a breakage in the system hardware. After observing a soft error, there is no implication that the system is any less reliable than before.
An error occurrence in a computer's memory system that changes an instruction in a program or a data value. Soft errors are typically remedied by error correction routines and do not typically damage a system's hardware; the only damage is to the data that is being processed.
Soft errors can occur when the radioactive atoms in the chip's material decay. The decay may produce a particle that strikes a memory cell causing the cell to change its state. The soft error rate (SER) steadily increases as the process technology scales downward. A single event upset, such as a particle strike, may cause single-bit upset (SBU) or multiple-bit upset (MBU). As the process geometry shrinks, the same particle will have a larger footprint in terms of bits. Therefore, MBU in nearby cells become more and more frequent.
Soft errors may also occur when the data being processed is affected by a noise phenomenon, typically when the data is on a data bus. The system may try to interpret the noise as a data causing errors in addressing or processing program code. The erroneous data may also be saved to memory and cause problems at a later time. Soft errors can occur on transmission lines, in digital logic, analog circuits, magnetic storage, and elsewhere, but are most commonly known in semiconductor storage.
When detected, a soft error may be corrected by rewriting correct data in place of erroneous data. Reliable systems use error correction to correct soft errors. However, in many systems, it may be impossible to determine the correct data, or even to discover that an error is present at all.
Soft errors are a major challenge in the design of memories and logic circuitry in microprocessors particularly as a result of technology scaling and increased SER.